Pressure-sensitive marking systems which involve localized contact between a color forming chromogenic compound, such as crystal violet lactone, benzoyl leuco methylene blue, etc. and a complementary color-developing substance, such as an acid-treated clay, to produce a color marking on paper or the like are well-known. Generally, colorless chromogenic substance is dissolved in minute oil droplets and encapsulated within the walls of pressure-rupturable microcapsules to prevent reaction between the chromogen and the color developer until the capsules are ruptured under the pressure of a writing device.
Two distinct systems have emerged, namely, the "transfer copy system" and the "self-contained" or "autogenous" system. In the transfer copy system, microcapsules containing the colorless chromogenic substance are coated in an aqueous dispersion of microcapsules onto the back of a substrate to form a "CB" (coated back) sheet which is superimposed onto the receiving sheet, which is coated with an electron-accepting color developer material, for example, of the Lewis acid type, such as acid-treated clay known as a "CF" (coated front) sheet. Upon application of localized pressure to the top side of the microcapsule-coated sheet, the walls of the microcapsules, which are on the back or underside of the CB sheet, are ruptured releasing the colorless, chromogenic substance for reaction with the acidic co-reactant on the CF sheet to provide a distinctive mark.
In the self-contained or autogenous system, the acidic co-reactant and the encapsulated chromogenic material are coated together to form an autogenous layer on the same substrate, and thus when the capsule walls are ruptured, there is no physical transfer or movement or either mark-forming component from one sheet to another. Rather, the colored mark is produced on the sheet bearing the capsules.
Typical record systems involved a multiplicity of sheets wherein, for example, the top sheet is a "CB" sheet comprising a substrate having a microcapsular coating on the underside thereof, superimposed upon a number of "CFB" (coated front and back) sheets, each of which comprise a support bearing a color-developing electron-accepting layer on the front thereof and a microcapsular coating on the underside thereof. The final or bottom sheet is normally a "CF" sheet bearing an electron-acceptor layer on the front side thereof. Thus, to form the CFB or middle sheets of the multi-ply record system, two coatings, i.e., a front and a back coating, are required for each CFB sheet. In a six-part form set, for example, ten separate coatings would be required. In view of the high energy costs involved in high temperature drying water-based coatings, it would be highly desirable to reduce the number of coatings required in the production of such multi-ply form sets.